A few friends and I are currently in debate about space. They say that there is no sound in space and that it is because there is no air in space. For instance if someone were talking to you, you couldn't hear what they were saying. I found it hard to believe either of those claims. I argued that there has to be air out there and that even if there was no air, there would still be sound because things like radio waves and light waves travel through space. Could you please clear us up on this argument.

Answer by Dave: I'm afraid that your friends are right. In empty space, there is no air, and what we call "sound" is actually vibrations in the air. Now, like you've said, there are indeed light waves and radio waves in space, but these waves are not sound, but light. Light does not need air to travel, but then you don't hear it; you see it, or it is interpreted by your radio set and then translated into sound.

Astronauts in space do talk to each other. In the spacecraft, there is plenty of air, so they just talk normally. When they are spacewalking, they talk by means of radios in their helmets. The radio waves, again, have no problem in space, but they're not sound. They're radio, which has to be converted into sound by the astronauts' headsets.

But can't there be vibrations in matter that isn't air? And if there are gases in space, then why can't sounds move through them?

Answer by Lynn: You're right that there are gases in space, and it's true that these gasses can propagate sound waves just like Earth's air allows sound to travel. The difference is that interstellar gas clouds are much less dense than the Earth's atmosphere. (They have fewer atoms per cubic foot.) So if a sound wave was traveling through a big gas cloud in space and we were out there listening, only a few atoms per second would impact our eardrum, and we wouldn't be able to hear the sound because our ears aren't sensitive enough. Maybe if we had an amazingly large and sensitive microphone we could detect these sounds, but to our human ear it would be silent.

There can also be vibrations in matter that's not gaseous: for example, the solid Earth or even the Sun (see the related link below). But although sound can travel through Earth, it can't travel from Earth to Mars because there's essentially no matter (gases, liquids, solids) in between the two planets for it to travel through.

So it's not strictly true that no sound vibrations can travel through space at all, but it is true that humans would not be able to hear any sounds in space.

But in movies when they show a large space ship exploding and another spaceship nearby they often play a large exploding sound. I'm wondering in large explosions (maybe not as small as a spaceship exploding, but say in a supernova) could a person hear the sound because possibly the explosion releases gases in which the acoustic energy is transported through the vacuum between the explosion and some observer in a spaceship (or possibly on earth) if the supernova or spaceship explosion was relatively nearby?

Answer by Lynn: I know in movies a lot of times they play sounds when things explode, but I don't know of any cases where this would actually be realistic. Because space is a vacuum, gases released into space expand very quickly, and as they expand their density decreases.

So say you were in a spaceship in the middle of a big space battle and a nearby ship exploded. The exploding ship would release gases and technically sound could travel along with them. However, since space is a vacuum, these gases will spread out very rapidly and the density will drop off very fast with distance from the explosion. (If you think about it, the amount of air in the ship is probably not very large compared to the volume of space between two ships.) So by the time the explosion reached your ship nearby, any sounds carried by the gas would still be too faint to hear. It seems more likely to me that what you would hear would be the shrapnel from the explosion banging into the hull of your ship. As you point out, it depends on distance. If the your ship was directly next to the exploding ship, you would be more likely to hear something, but it would also be bad news for your ship and crew!

It's pretty much the same for a supernova. The gases from a supernova explosion expand rapidly, and the density will drop off fast. I'm not sure how close you would have to be to hear a supernova, because I'm not sure where you would have to be to get densities close to Earth atmospheric values, and you might need a computer simulation to tell exactly. But to get some idea of how the density of gas would drop off as you expand the material of a star, I did a really simple calculation. If you took a star 50 times the mass of the sun and distributed its mass over a sphere of space with a radius equal to the planet Mercury's orbital distance, the density would already be 10 times less than atmospheric density at sea level on Earth. Mercury is pretty close to the sun, and you wouldn't be able to hear sounds even at that distance! In reality, not all the star's mass is ejected into space, and the gas that is expelled has shock waves, which are compressed. But the basic idea is that you would have to be extremely close to get densities high enough to hear anything. So we won't ever hear a supernova explosion on Earth, for example. It's a little sad, but space really is silent.

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